Nanotechnology refers broadly to a field of applied science and technology whose unifying theme is the control of matter on the atomic and molecular scale, measured in nanometers, and the fabrication of devices within that size range. Examples of nanotechnology in modern use are the manufacture of polymers based on molecular structure, and the design of computer chip layouts based on surface science. Despite the great promise of numerous nanotechnologies such as quantum dots and nanotubes, real commercial applications have mainly used the advantages of colloidal nanoparticles in bulk form, such as suntan lotion, cosmetics, protective coatings, drug delivery, and stain resistant clothing.
Materials reduced to the nanoscale can suddenly show very different properties compared to what they exhibit on a macroscale, enabling unique applications. For instance, opaque substances can become transparent (copper); inert materials can become catalysts (platinum); stable materials can turn combustible (aluminum); solids can turn into liquids at room temperature (gold); insulators can become conductors (silicon). A material such as gold, which is chemically inert at normal scales, can serve as a potent chemical catalyst at nanoscales. Much of the fascination with nanotechnology stems from these unique quantum and surface phenomena that matter exhibits at the nanoscale.
In order to optimize and extend the application of metal nanoparticles, methods must be developed to control the assembly and organization of these materials. Assemblies of nanoparticles also provide optical and electronic properties that are distinct compared to individual particles or disorganized macro-scale agglomeration.
One approach for organizing nanoparticles is to control the composition of the ligand shell around the particles. Typically the ligand shell plays an important role in imparting functionality for specific applications of metal nanoparticles. In most cases, ligands in the shell are selected to chemically define the nanoparticle surface for stabilization in different environments (e.g. aqueous or organic solvents) or to provide attachment sites for probe molecules for sensing applications. Several strategies for controlled assembly of metal nanoparticle are based on tailoring the composition of the ligand shell. Most of these approaches have been based on limiting the number of reactive ligands in the shell in order to control the possible types of assemblies that can be formed. However, such approaches contain various problems including difficult purification, low yield, and size restrictions.
Due to the challenges associated with organized assembly, few methods have been successful in the well-controlled formation of nanoparticle assemblies. Several strategies have employed DNA molecules for nanoparticle assembly based on electrostatic interactions or sequence-specific base pairing. Alternatively, nanoparticle assemblies have been prepared by controlling the composition of the ligand shell using a place exchange process to produce divalent nanoparticles that were assembled by inter-particle covalent linkages. In general, these methods are limited in the size/type of particles that can be assembled and the ligands that can be used. In addition, approaches that involve preparation in organic solvents complicate application of the nanoparticle chains in aqueous-based applications.
As such, research and developmental efforts continue in the field of nanotechnology in the pursuit of new nano-materials, including nanoparticle assemblies, exhibiting unique properties.